1. Technical Field
The present invention relates to network terminals connected communicable with other network terminals via a network, network systems connecting a plurality of network terminals in a communicable manner via a network and time synchronization methods and time synchronization programs for synchronizing network terminals making up such a network system.
2. Background Art
Conventionally sensor network systems enabling mutual communication of a plurality of sensor nodes via a network have been used for the purposes such as hydrographic observation and seismic observation. A sensor node is disposed at each observation point. The sensor nodes are terminals equipped with a sensor function for measurement of phenomena to be observed and a communication function for communication with other sensor nodes. Then, measurement data by each sensor node is collected and processed for observation of a target.
Such a sensor network system is required to synthesize the sensor nodes for temporal matching of measurement data on phenomena to be observed between the sensor nodes. Available techniques for time synchronization to synchronize the sensor nodes includes RBS (Reference Broadcast Synchronization) described in Non-Patent Document 1, for example. RBS is a technique using a time synchronization server that broadcast-transmits a packet called a “reference packet” at regular intervals to allow clients (sensor nodes) to set their timers on the basis of information of the reference packet received, thus synchronizing the sensor nodes.
[Non-Patent Document 1] “Sensor Network Techniques” written and edited by Shigeru Ando, Yosuke Tamura, Yoshito Tobe, and Masaki Minami, Tokyo Denki University Press, Published on May 20, 2005 (Chapter 3, Protocol of sensor networks, 3.1. Basic techniques, 3.1.1 Time synchronization, 3.1.2 RBS, pp 95 to 99)